Project EDDIE: Lake Mixing Carey, C.C., J.L. Klug, and R.L. Fuller. 1 August Project EDDIE: Lake Mixing Module. Project EDDIE Module 3, Version 1. mixing.shtml. Module development was supported by NSF DEB

The heat in a lake comes primarily from solar heating. Other heat sources: Streams Air Ground Sub-surface inputs (hot springs)

Light decreases exponentially with depth

So, wouldn’t we expect temperature to have the same pattern, since that light is getting converted to heat? Yes, but that is not what we often find.

Why is measuring temperature at depth important? If the temperature is isothermal throughout the profile, we can assume that the water column is able to mix Why is mixing important? –Oxygen –Nutrients –Organisms and other particles Conversely, if the temperature is not isothermal, we assume that the water column is stratified

April Isothermal Jul Summer Stratification Aug Jun Sep Oct Jan Inverse Stratification ice Mar Isothermal, Spring Turnover Nov Fall Turnover May For a north temperate lake in the northern hemisphere:

Epilimnion Hypolimnion Thermocline Metalimnion Thermocline is the depth where temperature changes the most; depth controlled by solar radiation and wind-driven mixing (fetch)

Why is warmer water at the surface?

April Isothermal Jul Summer Stratification Aug Jun Sep Oct Jan Inverse Stratification ice Mar Isothermal, Spring Turnover Nov Fall Turnover May

Photo credit: Midge Eliassen Why ice floats… It’s all due to Hydrogen bonding!

Stability -- the degree to which lake stratification resists mixing by the wind Stability depends on the difference in density between layers Schmidt stability- quantity of work required to mix the entire volume of water to a uniform temperature How much wind energy is needed to mix the lake?

Which lake (A, B, C) has the greatest Schmidt stability?

Stability of A > C > B

We can classify lakes based on how often they mix per year Dimictic Monomictic –Warm –Cold Amictic Oligomictic Polymictic Mixing regime

Dimictic = two periods of mixing per year: summer stratification fall turnover (mixing) winter inverse stratification spring turnover (mixing) Typical of northern latitudes Must have winter ice cover

Dimictic Temperate Lake - Summer and winter stratification Spring SummerFall Temperature °C Surface Bottom Lake Depth Temperature °C Winter Temperature °C Slide courtesy of K. Webster

Dimictic = two periods of mixing per year: summer stratification fall turnover (mixing) winter inverse stratification spring turnover (mixing) Mountain Lake, VA; Horne and Goldman 1994

High-frequency (10 minute) temperature measurements from Lake Sunapee, New Hampshire JanJulOct AprJan Photos by M. Eliassen; figure from C.C. Carey; data courtesy of LSPA

Lake Sunapee’s stability over the year

Comparison of Schmidt Stability to temperature profile heat map. What factors might explain variation in Schmidt Stability during summer stratification?

Let’s focus on winter in Lake Sunapee… Modified from Bruesewitz, Carey, Richardson, Weathers (2015)

monomictic -- one period of mixing warm monomictic stratifies in summer and mixes all winter (no ice) cold monomictic stratifies in winter (under ice) and mixes in “summer” polymictic -- mix frequently throughout the year Where would you expect to find lakes with these different mixing regimes?

Amictic -- Never mixes. Always stratified. Always covered with ice. Antarctica. Lake Bonney, Antarctica (Photo courtesy of G. Simmons)

Are there lakes that are always “summer” stratified? Maybe, but they usually mix occasionally, so they are called oligomictic. Oligomictic = Thermally-stratified much of the year but cool sufficiently for rare short mixing periods. They occur in the tropics and since there is no cold season, they do not have a cold hypolimnion.

polymictic amictic cold monomictic warm monomictic dimictic oligomictic Usually warm monomictic transitional Modified From Hutchinson and Löffler (1956)

Lake mixing module goals 1.Interpret variability in lake thermal depth profiles over a year. 2.Identify lake mixing regimes based on figures of water temperature. 3.Compare and contrast lake mixing regimes across lakes of different depths, size, and latitude. 4.Understand the drivers of lake mixing and thermal stratification. 5.Predict how climate change will affect lake thermal stratification and mixing.

Lake Rotorua, NZ The buoys of GLEON : sensor platforms from around the world Lake Sunapee, New Hampshire (USA) Yang Yuan Lake, Taiwan Lake Taihu, China Lake Erken, Sweden Trout Lake, Wisconsin (USA) Lake Mendota, (WI, USA) Lake Paajarvi, Finland Slide courtesy of K. Weathers

Activity A Divide into groups, with at least one laptop per group and each group assigned to one lake Access the color temperature figure for your lake, Excel data files (separate tabs for each lake), and student instructions handout. Follow directions on handout for Activity A.

GLEON lake characteristics

LillinonahActon LacawacAnnie FeeaghMügglesee

Activity B Divide into groups, with at least one laptop per group and each group assigned to one lake Access the color temperature figure for your lake, Excel data files (separate tabs for each lake), and student instructions handout. Follow directions on handout for Activity B.

Discussion 1.What were the mixing regimes for each lake? 2.Which lakes had the highest Schmidt stability? What factors might relate to stability? 3.How would climate change affect stratification? 4.What are the implications of altered stability for the six study lakes?

Activity C How will lake thermal structure respond to altered climate? To answer this question, we will use a lake model called GLM (General Lake Model) in which we can manipulate air temperatures and explore the effects on lake mixing and stratification

Figure from Hipsey et al. 2014

Air temperature simulations We will add +3 o C and +5 o C to all air temperature observations for Lake Mendota, Wisconsin, USA during the ice-free period of 2011 We can then compare the resulting output from the baseline (simulated 2011) and +3 o C and +5 o C scenarios to see the effects on the Mendota thermal profiles over time Create a figure that shows the time series of Schmidt stability from the three simulations on the same plot

Lake Mendota 2011: No change Lake Mendota 2011: +3 o C Air temperature Lake Mendota 2011: +5 o C Air temperature

Discussion 1.Compare the thermal heat maps for 2011, o C and o C. How are they similar, and how are they different? 2.What are the effects of the 3 and 5 o C increases in air temperatures on water temperature over time at 0m? 20m? What limnological mechanisms might explain these patterns? 3.What are some of the assumptions that went into this model output? Are they realistic?

Discussion, continued 4.What is the effect of increased air temperatures on Schmidt stability? Why? 5.As air temperature continues to increase, are the effects on water temperature and stability likely to be linear? Why or why not? 6.What are the implications of higher temperature on lake oxygen concentrations? Phytoplankton? Zooplankton? Fish?